Born at Florence in 1535; died at Rome 27 April, 1605, on the twenty-seventh day after his election to the papacy. His mother, Francesca Salviati, was a daughter of Giacomo Salviati and Lucrezia Medici, the latter being a sister of Leo X. From his boyhood he led a life of piety and always had an earnest desire to enter the ecclesiastical state, but could not obtain his mother's consent. After her death he was ordained priest and somewhat later Grand Duke Cosimo of Tuscany sent him as ambassador to Pius V, a position which he held for fifteen years. Gregory XIII made him Bishop of Pistoia in 1573, Archbishop of Florence in 1574, and cardinal in 1583. Clement VIII sent him, in 1596, as legate to France where he did good service for the Church in repressing the Huguenot influence at the court of Henry IV, and helping to restore the Catholic religion. On his return to Italy he was appointed prefect of the Congregation of Bishops and Regulars. In 1600 he became Bishop of the suburbicarian Diocese of Albano, whence he was transferred to Palestrina in 1602. Alessandro was an intimate friend of St. Philip Neri with whom he spent much time in spiritual conversation and whose advice he sought in all important matters. When Alessandro was Tuscan ambassador at the court of Pius V Philip predicted his election to the papacy.

On 14 March, 1605, eleven days after the death of Clement VIII, sixty-two cardinals entered the conclave. Prominent among the candidates for the papacy were the great historian Baronius and the famous Jesuit controversialist Bellarmine. But Aldobrandini, the leader of the Italian party among the cardinals, made common cause with the French party and brought about the election of Alessandro against the express wish of King Philip III of Spain. King Henry IV of France, who had learned to esteem Alessandro when papal legate at his court, and whose wife, Maria de' Medici was related to Alessandro, is said to have spent 300,000 écus in the promotion of Alessandro's candidacy. On 1 April, 1605, Alessandro ascended the papal throne as Leo XI, being then seventy years of age. He took sick immediately after his coronation. During his sickness he was importuned by many members of the Curia and by a few ambassadors from foreign courts to confer the cardinalate on one of his grandnephews, whom he had himself educated and whom he loved dearly, but he had such an aversion for nepotism that he firmly refused the request. When his confessor urged him to grant it, he dismissed him and sent for another confessor to prepare him for death.